George59 and Ceciliabr, may I suggest you read (or read again) the tips in the help included with Sweet Home 3D? You might find a lot of useful information there! [;)]

In the graphics editors I inspired myself from, the mouse wheel doesn't zoom in / out without pressing a key (generally the same one as in Sweet Home 3D).

About magnetism on objects, there are already a few behaviors you probably didn't pay attention to:
- doors and windows are placed automatically on walls
- other objects don't cross walls
- objects placed on other ones are automatically elevated when they are added
- an object placed beside an other one won't overlap it if they intersect vertically and their bounding box intersect of a few pixels (go gently).

The selection color should come from the matching setting in your system. What system do you use?